added nzip.c
SDL_RWops functions for nzip
close files
properly report close errors
not a dummy iszip function
documentation
and free some memory on errors
free memory from rwops
remember to set the close function for rwops
zip up ndata with make install
change names refering to "pack" and "cache" to something more appropriate for zips
* Properly test for libzip.
* -lz should be provided by libzip dep now.
* Some stuff is no longer const and needs to be handled as such.